In a known way, electroluminescent display devices comprise an emission region formed from a matrix of pixels, each pixel typically consisting of a plurality of differently coloured (RGB: red, green and blue, in general) subpixels, and from an electrical connection region arranged to lie adjacent this emission region. Production of such microdisplay or microscreen devices, i.e. having pixels with side lengths of less than 50 μm, gives rise to many technical problems due to this targeted small pixel size and in particular to the use of OLED technology which, for each pixel, uses a light-emitting multilayer structure comprising an organic film intermediate between two, internal and external, electrodes that serve as anode and cathode, one of which is transparent or semitransparent to the emitted light whereas the other is generally reflective. For a description of such an OLED microdisplay, for example, mention may be made of document EP-A2-2 065 949 in the name of the Applicant.
In fact, OLEDs based on small molecules cannot be microstructured with conventional microelectronic technologies and it is therefore impossible for three separate emitting structures, emitting in the red, the green and the blue respectively, to be spatially deposited on one and the same internal base electrode so as to form the three subpixels of each pixel. The small pixel size requires that a white-light emitter be vacuum evaporated and then optically filtered, the subpixels being distinguished from one another by the internal base electrodes on which they are respectively deposited.
These base electrodes are generally separated from one another by an electrically insulating resin that covers the substrate, and very often, after a step in which the last metallization level corresponding to each base electrode is etched, a lithography step is carried out which softens the topology of the substrate thus covered but also allows the opening of each subpixel to be defined relative to the resin that partially covers the periphery of its base electrode.
This lithography step proves to be essential because the OLED layers are very thin and consequently very sensitive to breaks in slope (all the more so because the aforementioned vacuum evaporation is not a conformal deposition method).
However, one major drawback of this lithography is that the aperture of each subpixel (which increases as the coverage of the base electrode by the resin decreases) must be written in the unetched metal surface of this base electrode. This results in the effective area of the diode and, consequently, the aperture ratio of the display device (which is defined as the ratio of the actual emissive area of the display to its total area) being proportionally reduced.
It is for this reason that it is desired, at the present time, to increase as much as possible the aperture ratio of displays, whether polychromatic (i.e. defined by these subpixels) or monochromatic (i.e. only defined by pixels, there being no subpixels), using OLED technology because for a given luminance (expressed in cd/m2) of the pixels or subpixels the greater the aperture ratio, the lower the current density flowing through them and therefore the longer the lifetime of the display. This maximizing of the aperture ratio is particularly desired for microdisplays because, on the one hand, of their aforementioned small pixel size, and, on the other hand, of the use of colour filters which absorb a non-negligible part of the emitted luminous flux.